1. Field of The Invention This invention relates generally to monolithic, multi-channel photodetector amplifier arrays and circuit channels integrated on a semi-conducting substrate. More particularly, the invention relates to monolithic, multi-channel photodetector amplifier arrays and circuit channels integrated on a GaAs substrate having electrical insulation between photodetector and amplifier layers.
2. Background Description
Considerable attention has been given recently to optoelectronics and to photodetectors in particular because their high signal processing capabilities make them very useful in optical signal processing applications, in optical fiber communications, and in areas where optical interconnects between semiconductor chips are required as described in "GaAs Integrated Optoelectronics", IEEE Trans. Electron Devices, vol ED-29, pp. 372-1381, Sept. 1982, by N. Bar-Chaim et al. Optical processing techniques have the capability to perform parallel signal processing functions at rates two orders of magnitude or more greater than conventional signal processing techniques based upon present integrated circuit technologies. Optical processing systems consequently are very useful in a variety of military and commercial signal processing applications. Typically, applications such as spectrum analysis are at system front ends where real time signal processing functions are most demanding. Special microelectronic signal processing circuits are also needed with optical processors to provide an interface between the optical processing components and digital computers.
A significant problem to be solved in order to make full use of optical processing is efficient optical to electrical transduction including the capability to preserve essential signal characteristics and to reject unneeded data. For this purpose sensitive, high-speed, high-dynamic-range photodetectors, detector-amplifier single channel circuits, and integrated photodetector arrays followed by high-speed microelectronic signal preprocessors are needed.
Monolithic single channel detector-amplifier circuits, multiple channel detector-amplifier arrays, detector-multiplexer arrays, detector-preprocessor arrays, and other detector circuits and arrays which have one or more photodetectors together with other circuit components such as resistors, capacitors, and transistors, including field effect transistors (FETs), on the same semiconductor chip have significant potential for use in optoelectronic applications in communications, radar, acoustics, and electronic warfare systems. Optoelectronic integrated circuits which combine photodetectors with amplifier circuits on the same GaAs chip have the advantage of reducing the parasitic reactance between the optical detecting element and the electronic signal processing circuit. Such integration provides high performance, compact, and reliable components for high speed and/or high dynamic range optical systems applications.
However, fabrication of an integrated detector amplifier circuit is made complicated by the incompatibility between the material requirements of the photodetectors on the one hand and of the other circuit components, such as the FETs and resistors, on the other hand. To obtain low photodetector dark current and high quantum efficiency, it is desirable to build photodetectors on thick, low doped GaAs material, whereas FETs, for example, require thin layers with higher doping concentrations.
Previously, photodetector circuits including single channel detector-amplifiers and all photodetector arrays were fabricated with the low doped detector material being in direct contact with the highly doped transistor material. In these cases, active layers, which were electrically different and which may have been but need not have been chemically similar, were in direct contact with each other as described in Electronics Letters, Vol. 22 No. 14, page 753 (July 3, 1986). by D. K. W. Lam et al. Without electrical isolation between the photodetector and amplifier layers, unanticipated effects of one material on the components fabricated in the other material occur. These unanticipated effects often made it necessary to redesign the circuits, fabricate circuits having the modified designs, and reevaluate the modified circuits before optimum success could be achieved. In addition, due to the risks thus involved, the level of integration of reliable optoelectronic circuits which could be fabricated was limited.